It is well known that hydrocarbon feedtstocks containing higher molecular weight hydrocarbons can be converted into lighter weight hydrocarbons such as gasoline by the process of catalytic cracking. In modern fluid catalytic cracking units, the cracking reaction is effected by introducing the hydrocarbon feedstock at the lower, or upstream, end of a riser reactor pipe together with hot fluidized catalyst. The hot catalyst supplies all or a major proportion of the heat to vaporize the feedstock and to carry out the endothermic cracking reaction.
The vaporized feedstock and catalyst pass up the riser reactor together at high velocity. Because of the high activity of the catalyst, the cracking reaction has generally proceeded to the desired extent at the upper, or downstream, end of the riser reactor. The cracked hydrocarbons are then separated from the catalyst in a disengaging vessel and are sent downstream for further processing and/or storage. The catalyst is, in turn, stripped with an inert gas such as steam to remove entrained hydrocarbons before being sent to a regenerating zone for removal of the coke which built up on the catalyst during the cracking process. The regenerated cracking catalyst is then reintroduced into the riser reactor.
As mentioned above, modern fluid catalytic cracking units are designed so that the desired degree of catalytic cracking is attained at the downstream end of the riser reactor. If the cracking reaction proceeds beyond this point, overcracking occurs and undesirable, lighter weight products are formed. To avoid overcracking, it is very important to rapidly and completely separate the hydrocarbons from the catalyst. Furthermore, since thermal cracking proceeds even without the presence of catalyst, it is important to rapidly remove the hydrocarbons from the elevated temperatures within the disengaging vessel.
Many different designs for fluidized catalytic cracking units have been employed in an effort to achieve rapid and complete separation of the hydrocarbons from the catalyst and also to achieve rapid removal of the hydrocarbon products from the disengaging vessel. One such design is shown in FIG. 1. In FIG. 1, the riser reactor feeds directly into first-stage cyclone separators located within the disengaging vessel. For convenience, the riser reactor is shown feeding into only one first-stage cyclone; in practice there are several. The cracked hydrocarbon vapors exit out the top of the cyclones and pass to second-stage cyclone separators (again, only one is shown for convenience), while the catalyst exits out the bottom via standpipes, or dip legs, into the catalyst bed, or dense phase.
Upon exiting the second-stage cyclones, the hydrocarbons are sent downstream (as shown) for further processing and/or storage, or, alternately, they are sent to third-stage cyclones. Since the entire amount of catalyst flowing in the riser reactor enters the first-stage cyclones in this design, the cyclones are heavily lined with refractory to withstand the erosion caused by the catalyst. These cyclones are also designed with large diameter dip legs to accommodate the heavy catalyst loading.
In the fluid catalytic cracking unit shown in FIG. 1, overcracking is not a serious problem during steady-state conditions. However, catalyst surges are inevitable during the operation of a fluid catalytic cracking unit and overcracking does become a problem when such surges occur.
During steady-state conditions, the separation of the hydrocarbons from the catalyst occurs very quickly in the first-stage cyclones so catalytic overcracking is minimized. Furthermore, little catalyst is present in the dilute phase and this, too, helps minimize catalytic overcracking. A moderate amount of thermal overcracking occurs during steady-state conditions because all of the hydrocarbons have some residence time in the dilute phase as they pass from the first-stage cyclones to the second-stage cyclones before being removed from the disengaging vessel.
In contrast, catalyst surges cause serious problems of overcracking in the FIG. 1 unit. When a catalyst surge enters the first-stage cyclones, separation efficiency is greatly reduced and excessive amounts of hydrocarbons flow down the dip legs while excessive amounts of catalyst spew out the top of the cyclones into the dilute phase. The result is that hydrocarbon-catalyst contacting is increased in both the dense phase and in the dilute phase and the undesirable, lighter weight hydrocarbon products are formed.
Another fluid catalytic cracking unit is shown in FIG. 2. In FIG. 2, the riser reactor includes a flow reversal cap which directs the hydrocarbons and the catalyst downward into the dilute phase of the disengaging vessel and towards the dense phase. The hydrocarbons later change direction and flow into the cyclone separation system. As in FIG. 1, only one first-stage cyclone and only one second-stage cyclone are shown for convenience.
The FIG. 2 unit features a low catalyst loading on the cyclone system since only a small percentage of the catalyst is carried up from the dense phase by the hydrocarbons and stripping gas. Thus, erosion inside the cyclones is reduced and relatively small-diameter dip legs can be employed.
During steady-state conditions, a moderate amount of catalytic overcracking occurs since the hydrocarbons and the catalyst remain in contact in the dilute phase. Thermal overcracking occurs as well in the dilute phase during steady-state conditions. When a catalyst surge occurs, the amount of overcracking increases since even more catalyst enters the dilute phase.
Thus, there exists a strong need in the petroleum refining industry for a fluid catalytic cracking unit which features low catalyst loading on the cyclone system and which also keeps overcracking to a minimum, during both steady-state conditions and during catalyst surges. Within the past few years, two new designs have been suggested to eliminate these problems. As will be seen, both designs reduce the above-mentioned problems, but create new problems of their own.
FIG. 3 is a simplified drawing of a fluid catalytic cracking unit disclosed in Myers, U.S. Pat. No. 4,070,159, which is hereby incorporated by reference. In the Myers design, the cyclones are directly and laterally connected to the upper extremity of the riser reactor, the top of which is open into the disengaging vessel. The upward flowing catalyst is carried by inertial momentum into the dilute phase while the hydrocarbon vapors pass laterally into the cyclones. At the top of the vessel, the catalyst hits a conical deflector plate which directs it radially to the sides of the vessel. The catalyst finally drops by gravity to the dense phase at the bottom of the vessel.
The Myers apparatus avoids heavy catalyst loading to the cyclones because only a small percentage of the catalyst enters the cyclones. The design also reduces overcracking because most of the hydrocarbons are rapidly and effectively separated from the catalyst at the top of the riser reactor and in the first-stage cyclones, during both steady-state conditions and during surges. However, some overcracking still occurs because a small percentage of the hydrocarbons are carried into the dilute phase where both catalytic and thermal overcracking occurs. In fact, these hydrocarbons are likely to be seriously overcracked since their only route of exiting the dilute phase is by passing against the flow of material leaving the riser reactor.
The most serious problem associated with the Myers design is that of differential thermal expansion between the riser reactor and the cyclone system which occurs predominantly during start-ups and shutdowns. This differential can cause twisting of the hardware which can, in turn, result in the breaking of welds, the loosening of refractory lining, etc. To minimize the adverse effects of differential thermal expansion, the Myers apparatus contains an expansion joint, shown in FIG. 3 between the first-stage and second-stage cyclones. In such an application, expansion joints are unsatisfactory in several respects. For example, they are prone to fatigue failure from the lateral and longitudinal movement, catalyst erosion can wear through the metal since the joints cannot be refractory lined, and coke deposits can build up in the joints and can prevent movement.
FIG. 4 is a simplified drawing of the apparatus disclosed in Fahrig et al., U.S. patent application Ser. No. 096,939, filed Nov. 23, 1979, now U.S. Pat. No. 4,295,961 issued Oct. 20, 1981, which is hereby incorporated by reference. The Fahrig design features cyclones which are directly and laterally connected to the flow reversal cap of the riser reactor. By directing the hydrocarbon-catalyst mixture downward before separation, this design augments inertial momentum with gravitational momentum to ensure that an even more rapid and effective separation is made at the cyclone inlet. However, as with the Myers design, overcracking can be a problem since hydrocarbons which enter the dilute phase must exit by passing against the flow of material leaving the riser reactor.
The Fahrig apparatus is also an advance over the Myers apparatus because the problem of differential thermal expansion between the riser reactor and the cyclone system is reduced and the need for an expansion joint is eliminated. This reduction occurs because, in Fahrig, the cyclones are directly connected to the cap portion of the riser reactor rather than to the riser portion. It can be seen that the temperature differential between the cyclones and the cap will be less than the differential between the cyclones and the riser.
Therefore, a need still exists for a fluid catalytic cracking unit which offers the following four features:
(1) low catalyst loading on the cyclone system;
(2) rapid and effective separation of the hydrocarbons from the catalyst, during both steady-state conditions and during catalyst surges;
(3) rapid removal of the hydrocarbons from the elevated temperatures within the disengaging vessel; and
(4) elimination of the problem of differential thermal expansion between the riser reactor and the cyclone system.